Add canonical place about accessing shared global QField directories

[suricactus]

A good candidate to be rewritten and serve as common place for this docs: docs.qfield.org/how-to/outside-layers/#add-a-layer-outside-of-exported-project-folder

As discussed in #215

If you directly connect from QGIS to your database you can make use of a pg_service.conf file by placing it in the QField data folder. You can place your file either on the Internal Device Storage or on the SD Card Storage. You can check the path for the QField data folder in the bottom of the About QField screen in the app.
Usually the path on Android devices looks something like this: /Android/data/ch.opengis.qfield/files/QField.